/Engine/Source/Runtime/SlateCore/Public/Rendering/RenderingCommon.h
Called to figure out whether we can make new windows for popups within this viewport. Making windows allows us to have popups that go outside the parent window, but cannot be used in fullscreen and do not have per-pixel alpha.
virtual [FPopupMethodReply](API\Runtime\SlateCore\Input\FPopupMethodReply) OnQueryPopupMethod() const